1. Field of the Invention
This invention relates to a process for reducing the radioactive contamination in waste product phosphogypsum.
In the industrial production of phosphoric acid by the wet methods phosphate rock is reacted with concentrated sulfuric acid for simultaneously producing both phosphoric acid solutions and calcium sulfate products. The calcium sulfate, which may be recovered as the dihydrate or phosphogypsum, the hemihydrate, or the anhydrite is contaminated with most of the impurities originally present in the phosphate ores. In the past, these calcium sulfate products were generally discarded as unwanted by-products because of the impurities; and large ponds and piles of these materials can be found at most phosphoric acid plants.
In attempts to produce marketable products from the heavily contaminated gypsums, extensive purification operations and alteration of phosphoric acid production conditions have been attempted without practical success. Most of these attempts have focused on other impurities, and radioactive contaminants remain even after extensive washing and elutriation and other process modifications. Typical phosphogypsum from the Prayon and modified Prayon processes contains about 25 picoCuries per gram of radiation, measured as radium 226. This contamination has become of increasing concern to governmental regulatory agencies and the industry itself. The radium imparts a slight radioactivity to the gypsum which may be dangerous even in the minute levels involved in industrial or construction products made from that material. It is possible that limitations will be placed upon radioactive levels for further stockpiling of this phosphogypsum. For example 5 pCi/g has been proposed as an upper limit on phosphogypsum by the Environmental Protection Agency of the United States.
2. Description of the Prior Art
There has been little prior concern for reducing radioactivity of phosphogypsum. U.S. Pat. No. 3,949,047 and U.S. Pat. No. 4,282,192 disclose treatment of the monocalcium phosphate (MCP) solution prior to the precipitation of the calcium sulfate. In the former, the MCP is treated by addition of barium compounds to reduce radium contamination, and in the latter, the MCP is treated with a sequestering agent and a first calcium sulfate precipitate high in radium is discarded. U.S. Pat. No. 4,146,568 discloses a process for reducing radioactive contamination in the phosphogypsum itself by slurrying it with a dilute sulfuric acid containing barium sulfate and separating the solids resultant therefrom into fine and coarse fractions. It is indicated that the fine fraction predominates in the radioactive contamination. It is not indicated proportionally how much of the starting phosphogypsum is recoverable, but losses could be considerable.
Published European patent application No. 12,487 of June 25, 1980 discloses that phosphogypsum can be converted to anhydrite by treatment with 60% sulfuric acid at 60.degree. C. for 20 minutes. The obtained anhydrite particles are said to be 2.5 micrometers long and 0.5 micrometers thick.
From the above, there is still a need in the art for effective and economical means for removing a substantial portion of the radium that originates in phosphate rock. Furthermore, there is a need in the art for making calcium sulfate products that have acceptable levels of radium such that they may be utilized in wallboard and other industrial and construction materials. There is need at the present time to provide improved processes for reducing the radioactive contaminants in waste product phosphogypsum. The process of the present invention offers a solution to these needs.